Due to their parents' remarriage, Riri (Tsubasa Hachino) and Yuzuri (Koharu Saku) meet as step-sisters. Riri had always wanted a younger sister, and Yuzuri's arrival fulfilled that desire. However, her feelings gradually transform from simple sisterly affection into something far more intense and abnormal. As their bond deepens, their relationship evolves into one entangled with love, obsession, and twisted devotion.